1. Field of the Invention
This invention relates to light-emitting devices driven by an electric field and which are commonly referred to as electroluminescent devices. In particular, this invention relates to electroluminescent devices having a light emitter formed from an electroluminescent light-emitting material and an insulating material and which is capable of being driven by either a positive or negative electric field, that is, the device is capable of being used with either a forward or reverse electrical potential. Most particularly, the invention relates to a light emitter capable of being driven by an alternating electric field, that is, the light emitter is capable or being driven by the alternating voltages associated with an alternating current in either a forward or reverse current direction.
2. Background
Inorganic electroluminescence devices were discovered by Destriau in 1936. Destriau observed that when suitably prepared inorganic zinc sulfide phosphor powders activated with small additions of copper, were suspended in an insulator and an intense alternating electric field (15 kV) applied with capacitor-like electrodes, light emission resulted. Electroluminescent research gained further impetus with the advent of transparent conducting electrodes based on tin oxide (SnO.sub.2) in the late 1940's. Typically early devices were composed of two sheets of electrically conductive material serving as electrodes, one a thin conducting backing and the other a transparent conductive film, placed on opposite sides of a plastic or ceramic sheet impregnated with an inorganic phosphor, such as zinc sulfide doped with small amounts of copper or manganese. A transparent glass sheet placed next to the transparent conductive sheet served as an outermost protective substrate. Application of an alternating voltage to the electrodes caused an electric field to be applied to the phosphor. Each time the field would change, radiation of the wavelength of visible light was emitted. Although a large amount of research and investigation was conducted on the alternating current electroluminescent devices, the devices never achieved practical application although they were originally highly touted as a room lighting source. Unfortunately, at high brightness levels, the AC electroluminescent devices exhibited a very short life and after about 1963, most of the research into the AC electroluminescence devices was severely curtailed. The most recent efforts in this area have been directed to a molecular carbon (fullerene-60) system. The high voltage drive requirements, the associated high cost of drive circuitry, poor stability, and lack of color tunability have made these devices cost prohibitive.
Two other inorganic devices, 1) direct current (DC) inorganic semiconductor light emitting diodes (LEDs) and 2) fluorescent ion doped inorganic semiconductor thin film devices, trace their origins to the mid-fifties. Light emitting diodes based on forward biased inorganic semiconductor p-n junctions are limited to small area applications as a result of color, efficiency, and cost limitations. The other inorganic devices, fluorescent ion-doped inorganic semiconductor thin film devices, require high operating voltages to accelerate electrons or holes to sufficient energies in order to excite or ionize the fluorescent ion centers. Such high operating voltages result in thin-film instability and failure of such devices.
In the last decade, there has been an emerging interest in direct current (DC) molecular and polymer electroluminescence devices. A variety of organic molecules and conjugated polymers, copolymers, and mixtures have been found to exhibit electroluminescent properties. Light-emitting diodes incorporating these materials have demonstrated all of the necessary colors (red, green and blue) needed for display applications. However, a need continues to exist to lower the device operating voltages and to increase their light-emitting (output) efficiency. Further improvements in charge injection and the balancing of charge transport are needed. Because of the asymmetry of the device configuration, efficient charge injection occurs only in one direction (forward DC bias). Under reverse bias, most of the devices either degrade quickly or show very poor performance. Although efforts have been made to improve the charge injection efficiency by the use of low work function electrodes such as calcium or magnesium and the use of an electron transporting material to improve negative charge (electron) injection, such devices continue to be operational only in one direction. Similarly, efforts have been made to improve charge injection efficiency by the use of high work function electrodes such as indium tin oxide (ITO) or gold and the use of hole transporting materials to improve positive charge (hole) injection. Such devices also continue to be fully operational in only one direction. A need continues to exist to develop bipolar devices that have low turn on and operating voltages, flexibility, large area, high operating (light output) efficiency and low production cost.
It is an object of the present invention to provide an alternating or reversible direct voltage drive electroluminescent device.
It is an object of the present invention to provide a low operating voltage electroluminescent device.
It is an object of the present invention to provide an electroluminescent device using electroluminescent organic materials including organic molecules and organic polymers.
It is an object of the present invention to provide a flexible electroluminescent device.
It is an object of the present invention to provide a large electroluminescent area device.
It is an object of the present invention to provide a device with a longer lifetime.
It is an object of the present invention to provide an electroluminescent device with high emissive light output.
It is an object of the present invention to provide frequency modulated light output.
It is an object of the present invention to provide amplitude modulated light output.